1. Field of the Invention
This invention relates to a method of producing a bonded wafer comprising the steps of implanting ions of a light element such as hydrogen or helium to a predetermined depth position of a wafer for active layer, bonding the wafer for active layer to a wafer for support substrate and then exfoliating the ion implanted portion through a heat treatment.
2. Description of Related Art
As the production method of the bonded wafer, there is known a so-called smart cut method comprising the steps of implanting ions of a light element such as hydrogen, helium or the like into a wafer for active layer at a predetermined depth position to form an ion implanted layer, bonding the wafer for active layer to a wafer for support substrate directly or through an insulating film having a thickness of not more than 50 μm, exfoliating the wafer for active layer at the ion implanted layer and thinning an active layer portion exposed by the exfoliation to form an active layer having a predetermined thickness. In this smart cut method, the portion of the wafer exfoliated after the bonding can be recycled as a wafer, which is different from the conventional bonding technique. By such a recycling can be used one wafer (wafer for the active layer) in the bonded wafer plural times, which leads the way to reduce the material cost. Also, the wafer produced by the smart cut method is excellent in the uniformity of film thickness, so that the smart cut method is noticed as a production method with future potential.
In the bonded wafer produced by the smart cut method, however, there is a problem that as a buried oxide film (insulating film) 3 becomes thinner, voids or blisters are easily generated at the bonding interface to deteriorate the yield as shown in FIG. 1. This is considered due to the fact that when the insulating film has a certain degree of thickness, gases generated by the exfoliation heat treatment can be incorporated into the buried oxide film, but when the buried oxide film becomes too thin, the volume of the gas to be incorporated is decreased and hence the gases generated in the exfoliation heat treatment can not be completely incorporated into the buried oxide film and the remaining gas 15 results in the occurrence of voids or blisters.
As a means for suppressing the occurrence of voids or blisters, it is useful to use a method wherein the thickness of the active layer is changed in accordance with the thickness of the insulating film or concretely the strength of the active layer is enhanced by diminishing the thickness of the active layer corresponding to a portion of the insulating film thinned. For example, as disclosed in JP-A-2004-259970, there is a method wherein the thickness of the active layer having a strength capable of suppressing the voids or blisters before thinning can be experimentally calculated with respect to the thickness of the oxide film to set conditions for obtaining a suitable thickness of the active layer in the implantation of light element ions.
In the method of JP-A-2004-259970, however, as the thickness of the oxide film is thinned to not more than 50 nm, the thickness of the active layer should be thickened over 750 nm, which means that the implantation depth in the ion implantation also exceeds 750 nm. If the ion implantation is carried out up to such an implantation depth, it is required to use an expensive apparatus capable of applying a higher voltage and also a polishing quantity for the subsequent thinning of the active layer becomes larger, and hence there is a fear that the uniformity of the thickness of the active layer is not sufficient.
For this end, it is desired to develop a method wherein the occurrence of voids or blisters can be suppressed without thickening the thickness of the active layer before the thinning even in the production of the bonded wafer having a thinner thickness of an oxide film (particularly not more than 50 nm) or the bonded wafer having no oxide film.